1A and 1B wires on NEMA 17 motors

Measure all six pairs across four isolated motor leads. Put one verified coil on 1A/1B and the other on 2A/2B; NEMA 17 does not define wire colors.

Run the checker See the measurement method

Run Checker Key Numbers Method Evidence Compare Risks FAQ

Interactive checkerFour-lead bipolar motor

Identify the 1A/1B coil pair with a multimeter

Number the four loose motor leads 1–4. With the driver off and the motor disconnected, measure all six possible lead pairs. Enter each resistance in ohms, or enter OL/open when there is no continuity.

empty state

Enter the six readings to identify both coils.

A simple four-lead bipolar motor has two positive resistance readings and four open cross-pairs. No wiring recommendation is shown until all six readings are entered.

Next action

Power off the driver, unplug the motor, then measure all six combinations.

Boundary: This checks only the continuity pattern across the six possible lead pairs. It does not verify winding health against the motor datasheet, insulation, driver pinout, current setting, or thermal fit.

Ask for pinout review Check 12V and 4-wire limits

Report summary

Core conclusions before you energize the driver

The safe answer is procedural: do not trust color alone, find both coil pairs, then treat 1A/1B polarity as a direction reference.

For frame-size and selection context, see the NEMA 17 stepper motor guide.

2 terminals

1A and 1B are one phase, not two separate motors

On common bipolar stepper drivers, 1A and 1B are the two ends of one H-bridge phase winding. The other winding belongs on 2A/2B.

Evidence: S4, S5, S6

4-wire scope

The checker applies to 4-wire bipolar NEMA 17 motors

Six-wire and eight-wire motors add center-tap or series/parallel choices, so this page treats them as out of scope unless configured as two isolated coils.

Evidence: S2, S6

No universal map

NEMA 17 does not guarantee a universal color code

NEMA 17 is a mechanical frame label. Vendor color sets can differ, so continuity testing beats memorized red/blue/green/black tables.

Evidence: S1, S3

1 swap

Swapping 1A and 1B usually reverses phase polarity

If coil pairs are correct but direction is reversed, swap one phase pair or invert DIR in firmware. Do not move wires under power.

Evidence: S4, S5

Decision	Use this page when	Do not use it when
Wire 1A/1B	The motor is a 4-wire bipolar NEMA 17 and you can measure isolated coil pairs.	The motor has six or eight wires and the winding mode has not been selected.
Reverse direction	The motor runs smoothly but moves opposite to command.	The motor buzzes or cross-pair readings show low ohms.
Release production map	Connector pinout, color order, current limit, and firmware direction are documented together.	The answer depends only on an unverified online color chart.

Methodology

Identify coils first; name polarity second

The checker uses a deterministic rule: in a 4-wire bipolar motor, the two real phase pairs have measurable resistance and unrelated pairs are open circuit. Driver labels then become a mapping problem, not a guessing problem.

Step 1

Power down and isolate

Turn off the driver and unplug the motor. Resistance readings through the driver or while powered are not valid for coil identification.

Step 2

Find the two low-ohm pairs

Measure all six combinations on a four-lead motor. Two disjoint pairs should show continuity and match the motor datasheet; the other four combinations should be open.

Step 3

Assign phases, then decide polarity

Put one measured pair across 1A/1B and the other across 2A/2B. Check current settings using the exact motor datasheet and driver carrier instructions before a low-current jog.

Step 4

Document the map

Record motor colors, connector pin order, driver labels, current limit, and direction setting so production builds do not re-learn the same pinout.

Data sources

Evidence used and what remains uncertain

Public sources support the driver label model, the multimeter workflow, and the non-standard color-code warning. Public cited sources do not establish one universal NEMA 17 color map.

Published June 6, 2026 · Technical sources last reviewed October 14, 2026.

Source	Key data	Decision value
S1: ASPINA NEMA 17 selection guide Checked 2026-10-14	Published 2026-02-25; defines NEMA 17 as a mounting-dimension class, not a performance specification.	Supports the page rule that NEMA 17 alone cannot identify which colored wire is 1A or 1B.
S2: StepperOnline multimeter troubleshooting guide Checked 2026-10-14	Updated 2026-08-17; says to compare each phase reading with the exact motor datasheet, with open or zero/significantly low readings treated as possible faults.	Supports the checker workflow: verify each phase as a measurable winding before judging driver labels.
S3: StepperOnline closed-loop wiring diagram examples Checked 2026-10-14	Shows A+/A-/B+/B- wiring labels and multiple color sets across product examples.	Shows that driver phase labels identify coil pairs while motor lead colors vary by product.
S4: Pololu A4988 carrier documentation Checked 2026-10-14	Documents an A4988 bipolar stepper driver with adjustable current limiting, an 8-35 V motor supply range, and a warning that powered motor disconnection can destroy the driver.	Adds two practical boundaries: current limiting matters more than motor voltage alone, and motor leads must not be moved while powered.
S5: Pololu DRV8825 carrier documentation Checked 2026-10-14	Documents the DRV8825 carrier motor outputs, its current-limit method, and differences from A4988 carrier setup.	Lets readers translate A+/A-/B+/B- and 1A/1B/2A/2B without assuming wire colors.
S6: TI DRV8825 datasheet Checked 2026-10-14	The driver is built for two H-bridge outputs; bipolar stepper wiring depends on assigning each winding to one bridge.	Grounds the method in driver topology: 1A/1B are one bridge phase, 2A/2B are the other.
S7: NEMA17Motor adjacent pages and internal cable reports Checked 2026-10-14	Existing NEMA 17 pages already cover frame size, current, 12V wiring, and 4-wire cable limits.	This page avoids duplication by narrowing to 1A/1B coil identification and driver label mapping.
S8: StepperOnline RMS vs. peak current guide Checked 2026-10-14	Updated 2026-08-17; explains that motor ratings and driver current settings may use RMS or peak values, which cannot be compared directly without matching conventions.	Supports the instruction to use the motor phase-current rating with the exact driver carrier setting method.

Key facts and their use

Each fact links to its source and a practical next step for wiring or commissioning.

New fact or data point	Source	Action
NEMA 17 corresponds to an approximately 1.7 inch / 42 mm square flange size, but not torque, electrical characteristics, step angle, motor length, or colors.	S1	Do not infer 1A/1B color from the phrase NEMA 17. Treat the frame size as mechanical compatibility only.
Driver boards use carrier-specific current-limit methods, and a driver setting may use peak or RMS current while motor ratings may use another convention.	S4, S5, S8	Use the exact motor phase-current rating and exact driver carrier instructions. Confirm current conventions and thermal limits before testing.
The DRV8825 carrier uses different current-limit math from the A4988 carrier, even where the two carrier families share a similar pinout.	S5	Translate label schemes by phase, not by color or connector position.
StepperOnline motor testing guidance, updated 2026-08-17, says to compare winding readings with the motor datasheet and flags open or zero/significantly low readings as possible faults.	S2	The checker finds continuity pairs; compare both measured phase values with the exact motor datasheet before commissioning.

Limits in the available evidence

Use the exact motor and driver documentation when a universal value or pin map does not exist.

Universal NEMA 17 wire-color code

Status: No reliable public standard found

Do not map red/blue/green/black to 1A/1B without exact SKU evidence and measured continuity.

One normal NEMA 17 coil resistance

Status: No universal value

The checker identifies the two continuity pairs. Compare both measured phase values with the exact motor datasheet to assess winding health.

Safe powered trial-and-error wiring

Status: Contradicted by driver documentation

The result cards and risk table instruct power-down before moving motor leads.

Need a SKU-specific pinout review?

Share the exact motor and driver models with your measured pair map so the connector and label assumptions can be checked.

Request pinout review

Comparison

Which identification method should you trust?

The most common failure is mixing three label systems: driver phase labels, motor harness colors, and connector positions. Keep those boundaries separate before comparing methods.

Label set	What it means	Verified fact	Decision	Source
A4988-style carrier	Coil 1 and coil 2 driver outputs, often surfaced as 1A/1B and 2A/2B on controller boards or carrier diagrams.	A4988 carriers drive one bipolar stepper motor with active current limiting; Pololu warns not to connect or disconnect the motor while powered.	Use one measured winding on 1A/1B and the other on 2A/2B. Set current from this exact motor and carrier documentation before motion.	S4
DRV8825-style carrier	The two outputs for motor winding A and the two outputs for winding B; follow the labels printed on the exact carrier.	The DRV8825 uses two H-bridge winding outputs; Pololu documents a different carrier current-limit method from its A4988 carrier.	Translate A+/A- to one phase and B+/B- to the other; do not copy A4988 VREF math onto DRV8825 without checking the carrier.	S5
Motor harness colors	Vendor-specific lead colors or connector pin order, not a NEMA 17 standard.	ASPINA states NEMA 17 covers mounting dimensions, while StepperOnline examples show phase labels with product-specific color sets.	Use colors only after matching the exact SKU, cable revision, and measured coil pairs.	S1, S3

Method	Speed	Confidence	Risk	Best for
Continuity test	5-10 minutes	High for coil pairing	Requires access to motor leads and a meter	Unknown NEMA 17 lead colors or reused motors
Vendor datasheet	Fast when exact SKU is known	High only when SKU and cable revision match	Wrong if cable harness was changed	Procurement-controlled builds
Color chart from another motor	Fast	Low	Can mix phases and make the motor buzz, stall, or heat	Only as a clue before measurement
Trial and error under power	Unpredictable	Unsafe	Hot-plugging or cross-wiring can damage drivers	Not recommended

Risk controls

Failure modes to clear before full current

Mixed phase pair

Signal: Motor vibrates, locks, or moves erratically

Mitigation: Re-test all six wire combinations and ensure cross-pairs are open circuit.

Wrong direction reference

Signal: Motor runs smoothly but moves opposite of command

Mitigation: Swap 1A/1B on one phase or invert firmware DIR, then document the final choice.

Driver damage during rewiring

Signal: Rewiring while VMOT or USB-powered driver logic is live

Mitigation: Power down fully before moving motor leads. Reconnect only after the map is stable.

Over-current during jog test

Signal: Current rating conventions are unclear, the motor exceeds its specified phase current, or either component heats quickly

Mitigation: Compare the motor phase-current rating with the exact carrier current-limit method using compatible peak/RMS conventions. Do not use power-supply input current as coil current; follow component and thermal limits.

Examples below are illustrative; the readings are not from NEMA17Motor hardware tests or customer cases.

A4988 board marked 1A/1B/2A/2B

Meter finds black-green at 2.2 ohm and red-blue at 2.1 ohm

Put black/green on 1A/1B and red/blue on 2A/2B. If direction is wrong, swap black and green or invert DIR.

Motor harness has no datasheet

One cross-pair reads low ohms after using an online color table

Do not power the driver. The current map mixes coils; redo the six-pair measurement and relabel leads.

Production cable revision changed

Same motor model, different JST harness color order

Keep the driver labels but re-qualify connector pinout. NEMA 17 frame size does not preserve harness order.

FAQ

Practical 1A/1B wiring questions

Direct wiring answers

What are 1A and 1B wires on a NEMA 17 motor?

They are normally the two ends of one bipolar stepper phase as seen by the driver. The matching second phase goes to 2A and 2B.

Is 1A positive and 1B negative?

Only in a relative sense. Stepper drivers reverse current through the winding, so 1A/1B polarity mainly sets direction reference.

Can I connect either coil pair to 1A/1B?

Yes, if the other coil pair goes to 2A/2B. Swapping the two phases or reversing one phase may change direction, but the motor should still run when pairs are correct.

What happens if 1A and 1B are from different coils?

The motor may buzz, stall, heat, or behave erratically because each H-bridge output is not driving one complete winding.

Testing and safety

How do I find 1A and 1B with a multimeter?

With power off and the motor disconnected, measure all wire pairs. A valid coil pair shows low resistance; unrelated wires read open circuit.

Can I test continuity while the motor is connected to the driver?

No. Isolate the motor leads first. Driver circuitry can distort readings and powered rewiring can damage the driver.

What resistance should a NEMA 17 coil show?

There is no single NEMA 17 value. Compare each measured phase with the exact motor datasheet; an open or zero/significantly lower-than-specified reading can indicate a fault.

Should cross-pair readings be open?

For a normal 4-wire bipolar motor, yes. If a supposed cross-pair reads low resistance, the phase map is wrong or the motor is not a simple 4-wire bipolar type.

Color codes and driver labels

Are red, blue, green, and black colors standardized?

No. Some vendors publish color maps, but NEMA 17 itself does not define wire colors. Treat color as a clue, not proof.

Is A+/A- the same as 1A/1B?

Both label sets commonly identify the two ends of one phase, but the exact terminal mapping depends on the driver board. Check its pinout instead of matching names or colors by assumption.

Is B+/B- the same as 2A/2B?

They commonly identify the second phase pair. Check the exact driver pinout because terminal names and order vary by board.

Can I use firmware instead of swapping wires?

Yes. If the motor runs smoothly but direction is reversed, firmware DIR inversion is often cleaner than changing the harness.

Does a valid coil map mean the current setting is safe?

No. The checker only identifies the continuity pairs. Check the motor phase-current rating against the exact driver carrier instructions; current conventions and thermal limits differ by model.

Scope limits

Does this apply to six-wire NEMA 17 motors?

Not directly. Six-wire motors include center taps; you must decide whether to use end wires only or another wiring mode before applying 1A/1B labels.

Does this apply to eight-wire NEMA 17 motors?

Only after choosing series or parallel winding connections. Eight-wire motors need an extra configuration step.

Can wrong 1A/1B wiring burn the motor?

A mixed phase may not immediately burn the motor, but it can cause heat and driver stress. Over-current settings are the larger thermal risk.

What should I send for a pinout review?

Send motor model, driver board, wire colors, connector photos from both sides, all resistance readings, supply voltage, and current limit.

Next step

Need the pinout checked before a build or purchase?

Send the motor model, driver board, connector photos, wire colors, resistance readings, current limit, and target motion. We can review the map against the driver and cable boundary.

Request pinout review Check 4-wire cable limits Size 2-in-1 series cables

Before the first powered test

Confirm both winding resistances against the motor datasheet, use the exact driver carrier pinout and current instructions, and keep power off while changing motor leads.

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